A conventional semiconductor device control apparatus is used for controlling various motors. One example is a motor driving control apparatus 1 shown in FIG. 5. In this control apparatus 1, an input signal processing circuit 3 receives a rotation speed command signal in the form of a pulse-width-modulated (PWM) signal from an electronic control unit (ECU) 2 and converts it to a voltage signal corresponding to the duty of the PWM signal. This voltage signal is applied to a PWM control circuit 4. A correction command signal is also applied from a power source voltage correction circuit 5. The ECU 2 receives, for instance, an output signal of a coolant temperature sensor (not shown), which detects coolant temperatures in a radiator, and outputs the rotation speed command signal in accordance with the detected coolant temperature.
A PWM signal generation circuit 6 generates a PWM control command signal based on the voltage signal and the correction command signal. A triangular signal generation circuit 7 generates a triangular signal (carrier wave) in the form of a triangular shape at a fixed frequency (e.g., 19 kHz). The circuit 6 compares the PWM control command signal with the triangular signal by its comparator (not shown) to produce a resultant PWM signal. A driver circuit 8 receives this PWM signal and applies a driving signal to the gate of a N-channel MOSFET 9 in accordance with the PWM signal.
A series circuit of the FET 9 and a motor 11 is connected in parallel to a battery 10 mounted on a vehicle. The FET 9 is on the ground side and hence the apparatus 1 is constructed as a low side driving system. The motor 11 is used to drive a fan 11a of a radiator (not shown). The motor 11 is connected between terminals 1a and 1b. A noise filter 12 is constructed with a coil 12a and capacitors 12b and 12c as a π-type filter. The correction circuit 5 is connected to the terminal 1b, that is, the drain of the FET 9 to be responsive to a terminal voltage of the motor 11. Thus, the correction circuit 5 produces the correction command signal, which is varied with variations in a voltage of the battery 10.
A diode 13 is connected between the drain of the FET 9 and the input side of the noise filter 12. This diode 13 provides a path to allow a delay current to flow the battery side when the FET 9 turns off. The FET 9 generates noise signals when it repetitively turns on and off in response the PWM signal of 19 kHz. The noise filter 12 restricts those noise signals from being applied to the battery side.
The carrier wave frequency of the PWM signal is set to 19 kHz, which is near the upper limit of the audible frequency range, so that the sounds generated when the motor 11 is PWM-controlled become offensive to ears. For this reason, the noise filter 12 is necessitated for countering to the switching noises. Since the coil 12a as well as the capacitors 12b and 12c are large in size, the noise filter 12 occupies a considerably large mounting space. Further, since the switching frequency of the FET 9 is high, switching loss is large and heat generation of the FET 9 is large requiring a large heat sink. Thus, the apparatus 1 becomes large in size in the end.
For countering to this drawback, JP P2002-142494A proposes to set the carrier wave frequency of the PWM signal for driving the fan motor 11 to a very low frequency, which may be less than several tens (Hz). With this proposed very low frequency, hissing sound generated when a fan 11a rotates is reduced. On the contrary, vibration sound is remarkably increased due to very low speed rotations of the fan 11a and becomes offensive to ears.
In place of the PWM control, as shown in FIG. 6, the fan motor 11 may be driven with analog voltages by a control apparatus 14. This apparatus 14 includes an analog driver circuit 15 to control the analog voltage applied to the fan motor 11. The analog driver circuit 15 generates the gate voltage for the FET 9 by regulating the voltage signal applied from the input processing circuit 3 to a level suitable to driver the FET 9. No noise filter nor diode is provided in the control apparatus 14. Since the FET 9 is driven to operate in a linear region (unsaturated region) and a current continuously flows through the FET 9 during the motor control period, the heat generation by the FET 9 is large and a large heat sink is necessitated.